158 CONVERGENCE OF CHARACTER. [Chap. IV. 



seems at first sight no limit to the amount of profitable 

 diversification of structure, and therefore no limit to 

 the number of species which might be produced. TVe 

 do not know that even the most prolific area is fully 

 stocked with specific forms : at the Cape of Good Hope 

 and in Australia, which support such an astonishing 

 number of species, many European plants have become 

 naturalised. But geology shows us, that from an early 

 part of the tertiary period the number of species of 

 shells, and that from the middle part of this same period 

 the number of mammals, has not greatly or at all 

 increased. What then checks an indefinite increase in 

 the number of species ? The amount of life (I do not 

 mean the number of specific forms) supported on an area 

 must have a limit, depending so largely as it does on 

 physical conditions ; therefore, if an area be inhabited 

 by very many species, each or nearly each species will 

 be represented by few individuals; and such species 

 will be liable to extermination from accidental fluctua- 

 tions in the nature of the seasons or in the number of 

 their enemies. The process of extermination in such 

 cases would be rapid, whereas the production of new 

 species must always be slow. Imagine the extreme case 

 of as many species as individuals in England, and the 

 first severe winter or very dry . summer would exter- 

 minate thousands on thousands of species. Eare species, 

 and each species will become rare if the number of 

 species in any country becomes indefinitely increased, 

 will, on the principle often explained, present within a 

 given period few favourable variations ; consequently, 

 the process of giving birth to new specific forms would 

 thus be retarded. "When any species becomes very rare, 

 close interbreeding will help to exterminate it ; authors 



